It is known that many of liquid-phase reactions using gaseous reactants are accompanied by the evolution of heat. As a technology for controlling the reaction temperature of such an exothermic liquid-phase reaction, it is common practice to provide the reactor with a cooling jacket or an internal coil for circulation of a cooling medium and adjust the temperature or flow rate of said cooling medium so as to control the reaction temperature. With this technology, it is difficult to control the reaction temperature when its change is sharp and abrupt and, moreover, the efficiency of heat removal is low. Particularly, when the reactor volume is large, it is difficult to control the reaction temperature efficiently.
As the exothermic liquid-phase reaction using a gaseous reactant, for example, Wacker oxidation, so-called oxo reaction, the reaction giving a carboxylic acid or carboxylic anhydride from an alcohol and/or its ester and carbon monoxide, and the reaction giving a carbonic ester from an alcohol, carbon monoxide and oxygen are known.
The technology for the production of carbonic esters, by which an alcohol is allowed to react with carbon monoxide and oxygen in a liquid phase, can be roughly classified into a process using a copper compound as the main catalyst and a process using a palladium compound as the main catalyst.
Processes using a copper compound as the main catalyst are described in Japanese Patent Publication No. 11129/1970 and Japanese Patent Publication No. 58739/1985. Because the catalytic activity of any copper compound is comparatively low, these processes require the use of a copper catalyst in a high concentration, e.g. the order of several moles/L (tens of % on a weight % basis), in order to realize a practically acceptable reaction rate. However, since this procedure entails the presence of active species oxidative divalent copper ions and chloride ions in high concentrations in the reaction system, not only the reactor body but also the instruments, piping, valves and other hardware exposed to the reaction mixture are subject to severe corrosion.
Processes using a palladium compound as the main catalyst are disclosed in Japanese Patent Publication Nos. 8816/1986 and 43338/1986 and Japanese Patent Laid-open No. 287062/1989. In these processes, a weak acid salt or halide of copper and a weak acid salt or halide of an alkali metal or alkaline earth metal are used as promoters. Since the catalytic activity of any palladium compound is high as compared with copper compounds, a sufficiently high reaction rate can be realized even when the amount of the palladium species is as small as about one-thousandth compared with the copper species used in the first-mentioned processes and the amount of divalent copper ions used concomitantly for reoxidation of palladium can also be reduced to about one-tenth to one-hundredth as compared with the first-mentioned processes. However, since the oxidative divalent palladium and divalent copper ions, which are catalyst active species, exhibit high oxidizing activity even at low concentration levels, the reactor and ancillary equipment are still exposed to highly corrosive conditions.
Several processes have so far been proposed for producing carbonic esters in the liquid phase with the aid of such catalysts.
By way of illustration, the specification of EP-A 134668 discloses a continuous process for producing a carbonic ester which comprises condensing the gas from the reactor vapor phase and recycling a portion of the condensed carbonic ester to the reactor to control the concentrations of water and alcohol in the reaction mixture at low levels, minimize the deactivation of the catalyst and prevent side reactions.
Japanese Patent Laid-open No. 99041/1991 discloses a continuous process designed for prevention of corrosion which comprises feeding an excess of carbon monoxide/oxygen-containing gas to the reactor, with-drawing the product carbonic ester, by-product water and unreacted alcohol as an azeotropic mixture, subjecting this vapor to gas-liquid separation and recovering the carbonic ester from the condensate. It is mentioned that, in this process, the non-condensible gas from gas-liquid separator may be returned to the reactor.
Furthermore, the specification of EP-A 460732 discloses a continuous process for producing dimethyl carbonate which, for an improved yield of the carbonic diester, comprises bubbling a carbon monoxide-containing gas through the reaction mixture to encourage the evaporation of methanol, water and dimethyl carbonate from the reaction mixture and recovering water and dimethyl carbonate from the resultant mixture gas, characterized in that the concentrations of methanol and water in the reaction mixture are controlled within specified ranges. In this process, the non-condensible gas containing a large proportion of carbon monoxide is recycled to the reactor.
In these conventional processes for producing carbonic esters, control of reaction temperature is generally carried out in the ordinary manner described hereinbefore, namely by the method of circulating a cooling medium through the reactor jacket or coil. Accordingly, the efficiency of heat removal is low and it is often difficult to control the reaction temperature. Moreover, since a corrosive catalyst is used for the reaction in these processes, an expensive corresion-resistant material must be specified for the cooling coil and other hardware. Furthermore, in cases where the internal wall of the reactor is lined with a corrosion-resistant material such as glass and Teflon for added resistance to corrosion, passing a cooling medium through the jacket for cooling tends to cause cracks in the lining or a peeling thereof.
As an alternative technology for controlling the reaction temperature in the process for producing carbonic esters, a method is known which comprises cooling the reaction mixture in a heat exchanger and recycling it at a controlled temperature or flow rate. However, this method requires an additional capital expenditure for ancillary equipment. Moreover, it is essential that the circuit for the reaction mixture be made of an expensive corrosion-resistant material in order to prevent the corrosion by the catalyst in the reaction mixture. In addition, when the reaction mixture is circulated in this manner, the residence time is liable to vary so that the objective product cannot be obtained with good reproducibility.
In the prior art literature describing processes for producing carbonic esters, the concept of recycling a non-condensible gas to the reaction system is shown but the concept of controlling the reaction temperature by such recycling is not suggested.
Control of the reaction temperature in Wacker oxidation, oxo reaction, the reaction giving a carboxylic acid or carboxylic anhydride from an alcohol and/or its ester and carbon monoxide and the like has been also performed by the similar methods to those in said processes for procucing carbonic esters. Accordingly such problems as described above accompanying the reaction giving carbonic esters are also the case for above reactions.